This invention relates to a turbo machine and more particularly to an integral composite airfoil and disc assembly useable in the compressor or turbine components associated with such an engine.
For many years, attempts have been made to improve the performance characteristics of airfoils rotating within a gas turbine engine. One general approach provided for the use of composite materials in structuring the airfoil. Typically, this approach uses high strength elongated filaments composited in a light weight matrix. Recent efforts have introduced the use of boron, graphite and other synthetic filaments which have extremely high strength characteristics as well as high module of elasticity for compatibility with stiffness requirements of airfoils. To a large extent, composite airfoils have proven to exhibit performance characteristics equal to or better than airfoils homogenously comprised of metal. Furthermore, composite airfoils have been shown to offer significant weight reductions over conventional airfoils.
However, prior art composite activity has focused primarily, if not solely, upon constructing only the airfoil from a composite material. Specifically, the airfoil, constructed as a discreet component, has been inserted and locked in place in the periphery of a metallic compressor or turbine disc by conventional dovetail tang and dovetail slot arrangements. While acceptable for many applications, these assemblies are not entirely satisfactory where manufacturing costs are an especially significant factor or where stringent performance requirements must be met within restrictions on allowable weight. For these latter applications airfoil and disc assemblies manufactured entirely from composite materials appear to be highly suitable. Additional advantages may be realized if the composite airfoil and disc assemblies are manufactured as an integral unit.
The design and fabrication of a composite integral airfoil and disc assembly presents a number of difficult problems. Particularly, a major consideration involves adapting the unidirectional strength characteristics of composite filaments to the multi-directional stress field exhibited in the airfoil and disc assembly. Generally the matrix material, which is the composite material disposed between filaments, is usually much weaker than the composite filaments themselves. Consequently, orientation of filaments in directions compatible with the stress fields found in the assembly is of paramount importance in the design of the composite integral assembly. Achieving compatible orientation, however, is a difficult task. By way of example, filaments oriented in a direction compatible with the stress field of the airfoil are not usually compatible with the stress field associated with the disc portion of the assembly. Additionally, since each airfoil is displaced angularly from each other airfoil in the assembly, orientation of all the filaments in a direction compatible with the strength characteristics of one airfoil may not be compatible with the stress fields of other airfoils in the assembly. Hence, orientation of the filaments for compatibility with the stress fields of the disc and each airfoil is a significant design objective.
Another problem, which must be addressed during design of an integral composite airfoil and disc assembly, relates to the shear forces induced between layers of composite cloth or plies. In the manufacture of composite articles, high strength filaments are first bundled together in groups of approximately 1,000-10,000 filaments. A first set of bundled filaments are disposed in a first or warp direction (a direction parallel to the length of the cloth) and then interwoven with a second set of bundled filaments disposed in a second or fill direction (generally perpendicular to the warp direction). The resulting weave is commonly referred to cloth, fabric or plies. The composite article is then constructed by arranging one ply atop another ply until sufficient thickness is attained to form the article. A matrix material is then infiltrated into the aggregation of plies and subjected to heat and pressure to form a rigid composite article. The resulting fused article is comprised of layers of filaments between which is disposed matrix material. Loads induced on the article in an operating environment are resisted primarily by the filaments. However, the loads also induce shear forces between layers of cloth which must be resisted by the matrix material in shear. Hence, matching of the shear and strain characteristics of the matrix material with the shear and strain characteristics of the filaments is a significant design objective.